The parties to the statement further rejected ISIS's statement that its leader is now the ruler of the Muslim world Sunday.

ISIS has been successful in overtaking territory in both western Iraq as well as Syria, but continues to fight other rival Islamist fighters in Syria, said the Reuters report.

The statement indicated that the ISIS declaration of the caliphate may be "used as a pretext by foreign powers...to tile the balance against [Syrian] rebels" in their fight against Assad and his regime. Such would in effect "improve his image in the West as a legitimate leader," said the Reuters report.

"This announcement will only deepen the conflict with the central Al-Qaeda organization over the legitimacy of who represents it now and as a result who represents true Islam," said Jordan-based Hassan Abu Haniyah, an expert on jihadist Islamic groups to Reuters.

Al Qaeda denounced ISIS earlier this year, saying that its only representation within the Syrian conflict going forward would be the Nusra Front after tensions arose between Al Qaeda frontman Ayman Al Zawahiri and ISIS leadership, namely Abu Bakr Baghdadi.